Wideband phase-splitter



Dec. 13, 1955 R c, CHEEK 2,727,141

WIDEBAND PHASE-SPLITTER Filed July 22, 1950 C c O o :1 0 E 5* 6 D L65'67-69 4," r l l L WITNESSES! INVENTOR ATTORN EY Robert C.Cheek.

United States Patent WIDEBAND PHASE-SPLITTER Robert C. Cheek, Irwin, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application July 22, 1950, Serial No. 175,429

2 Claims. (Cl. 25027) My invention relates to communication apparatus and has particular relation to single side-band communication systems.

A single side-band system which typifies the prior art of which I am aware is shown in Patent No. 2,476,880 to B. E. Lenehan. The transmitter of a single side-band communication system such as is disclosed by Lenehan includes a pair of phase-splitters 5 and 21, one associated with the carrier oscillator 1 and the other with the audio output 15 and 17. Through each of these phase-splitters two components substantially equal in amplitude, and displaced by 90 in phase are supplied to the modulators of the transmitter.

The communication system disclosed by Lenehan operates on the whole satisfactorily. It has proved particularly useful in the transmission of speech. On occasions the desirability arises to communicate music or other information requiring greater frequency range than speech over a single side-band carrier system. This desirability may arise, for example, when a music service such as is supplied by Musak is communicated by carrier current. I have found that for such service the Lenehan system is not satisfactory.

it is accordingly an object of my invention to provide a single side-band communication system capable of transmitting music and other intelligence requiring a wide band of frequencies.

Another object of my invention is to provide a high fidelity single side-band communication system.

A further object of my invention is to provide a transmitter for a single side-band communication system of such design that the system shall be capable of transmitting signals having frequency components extending over a wide range of frequencies.

An ancillary object of my invention is to provide a phase-splitter capable of delivering at its output terminal over a wide frequency band outputs substantially equal in amplitude and displaced in phase by an angle which does not depart materially from 90.

My invention arises from the realization that the audio phase-splitter of the modulator of Lenehans transmitter is inadequate for the transmission of high fidelity signals such as that involved in the communication of music. Lenehans audio phase-splitter comprises essentially a series network consisting of a resistor 81 and a capacitor 77 supplied from the secondary of the audio transformer E9. The output of one of the phases is derived between the conductors 23 and 25; that is in efiect between the junction of the upper terminal of the secondary and the resistor 81 and the junction of the capacitor 77 and the resistor 81. The output of the other phase is derived between the conductors 27 and 25, that is in efiect, between the junction of the capacitor and the lower terminal of the secondary and the junction of the capacitor and the resistor.

I have found that this phase-splitter may be designed to operate without excessive distortion over a limited band, for example, over the ordinary speech frequency hand between 300 and 3000 cycles per second. Under such circumstances the phase-splitter has a midband requency equal to the geometric mean between the lower and higher frequencies. For the 200 to 3200 cycle band,

this midband frequency is equal to /200 X 3200 or approximately 800 cycles per second. At frequencies outside of the band-in the example assumed below 200 cycles per second or greater than 3200 cycles per second- Lenehans phase-splitter introduces substantial distortions and large inequalities between the output voltages at different frequencies develop. The phase displacement of the signals of frequencies outside of the band at the outputs of the phase-splitter also departs appreciably from 90.

in accordance with my invention 1 provide a single-side band communication system including phase splitting apparatus capable of delivering over a wide band of frequencies undistorted outputs having a phase displacement of substantially 90 and substantially equal amplitudes. in accordance with the broader aspects of my invention, this phase splitting apparatus includes a plurality of component phase-splitters each designed to operate over a separate component of the entire band over which the communication is to take place. For example, each of the phasesplitters may be designed to operate over an octave of the band, the octaves being so selected that the totality of the phase-splitters covers the whole communication band. The components of each phase-splitter are dimensioned so that the phase-splitter operates at a midband frequency corresponding to the geometric mean frequency of the octave. Thus a phase-splitter designed to operate between frequencies 1'1 and ;2 of a larger band are designed for a midfrequency equal to the 1 f2.

Each of the component phase-splitters is supplied through a filter having a pass band equal to the component band for which the phase-splitter is designed. The filter of the difi'erent phase-splitters are fractionally terminated, that is each filter is so dimensioned that in its stop band, in combination with other filters in their stop hands, it constitutes the input shunt component of the filters which is operating in its pass band.

Specifically, my invention is designed to apply to a single side-band system suitable for high-fidelity communication of music. Under such circumstances, a frequency band between and 8000 cycles should be comrnunicated without substantial distortion. This band may be divided into two octaves, one between 80 and 800 cycles and the other between 800 and 8000. I accordingly provide phase-splitting apparatus consisting of a pair of phase-splitters, one designed to operate without substantial distortion over the band between 80 and 800 cycles and having a midband frequency of 254 cycles and the other designed to operate over the band from 800 to 80-60 cycles and having a midband frequency of the order of 2540 cycles. The first of these phase-splitters is preferably supplied through a low pass filter capaole of transmitting Without substantial distortion frequencies up to 800 cycles per second. The other phase splitter is supplied through a high-pass filter capable of transmitting frequencies above 800 cycles per second. The components of the first filter are so selected that it constitutes in effect the inductive input to the high-pass filter and the components of the second filter are so selected that it constitutes the capacitive input component of the lowpass filter.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will readily be understood from the following description of a specific embodiment when read in connection with the accompanying drawing in which the single figure is a circuit dia-- gram of a preferred embodiment of my invention.

The apparatus shown in the drawing includes only a pair of component phase-splitters 1 and 3 designed to cover a predetermined frequency range. in certain situations the fidelity demanded may require more than two phase-splitters for fthes'arherange. in others, the frequency range to becomn unicated may be greater ti that which can be covered by two phase splitters and for this reason more than two phase-splitters may be required. Apparatus of either type including more than two phase-splitters lies within the scope of my invention.

The first phase-splitter 1 is designed to over the lower frequency band. Its principal elements are a capacitor 5, resistor 7, and winding 15 of reactor 17 connected in series. The junction of the capacitor 5 and the resistor 7 is grounded. The capacitor and resistor network (5, 7 is connected to input ter s 9 and 11 of the splitter 1 through a resistor 13 of reia ely high magnitude, which maintains current essentially independent of changes in reactance of the winding 15 and capacitor 5 with frequency. The capacitor 5 is connected to one of the output terminals 19 or" the phasesplitter 1 through the other coupled winding 21 of the reactor 1'7 an through a resistor 23. The resistor '7 of the phase-splitting network 1 is connected to the other output terminal 25 of the phase-splitter through an inductor Z7 and a capacitor 29. The output resistor 21, the coupled inductor winding 23 and the capacitor 5 of the network are dimensioned so that the network 21235 has substantially the same internal impedance as the network consisting of the resistor 7, the output inductor 27 and the output capacitor 29. Between one input terminal 9 and the opposite output terminal 25, a parallel network of an inductor 31 and a capacitor 3.5 are connected. These components 31 and 33 are dimensioned to correct the response of the phase-splitter at its extreme frequencies in the manner described by Lenehan in Patent 2,476,880.

The second phase-splitter designed to operate at the higher frequency band includes similar electrical components 35, 37, 39, 41, 43, 45, 47, 49, 5'1, 53, 55, 57, 59 and 61, respectively, dimensioned to correspond to the higher frequency band. The two networks are supplied in common from the unit 63 which is to supply the oscillations on which the phase-splitters are to operate. This unit may be the audio input unit of a music communication system, for example, a system consisting of a microphone or a phonograph pick-up and a high fidelity audio amplifier or any source of intelligence occupying a wide frequency band. Within the broad scope of my invention this unit 63 may also be an R. F. oscillator.

The input unit 63 is coupled to the first phase-splitter 1 through a T section filter consisting of a pair of inductors 65 and 67 between which a capacitor 69 is connected. The input unit 63 is coupled to the second phase-splitter 3 through a T section filter consisting of a pair of capacitors 71 and 73 between which an inductor 75 is connected. These T section filters constitute a frequency dividing network which is preferably of the fractionally terminated type. That is to say, the T section 6S6769 connected to the first phasesplitter 1 is designed to have, over the frequency range of the second filter 717375, in efiect the function of an inductor of such magnitude as to constitute the input component of the second filter and the second filter 717375 is designed to have, over the frequency range of the first filter, in effect the function of a capacitor of such magnitude as to constitute the input component of the first filter 65 6769. The equivalent inductive and capacitive components are shown in broken lines in the drawing. Preferably, the T section filters are so dimensioned that the first filter 65--6I69 is of the low pass type having a terminating frequency at the upper frequency of the phase splitter 1 and the 4 second filter 71-73-75 is of the high pass type having a terminating frequency at the lower frequency of the second phase-splitter 3. Within the scope of my invention, however, the filters may be of the band pass type and may be designed to pass only the bands over which the phase-splitters are to operate.

The output of the phase-splitters 1 and 3 is of the balanced type, each including a pair of phase terminals 19 and 25 and 47 and 53, respectively, which have an electrical center at ground. Each phase terminal 19, 25, 4-7 and 53 is connected to the control electrode 77, 79, 81 and 83, respectively, of a suitable amplifier 85, 87, 89. and 91, respectively. The anodes 93 and 95 and 97 and 99 of the respective amplifiers 85 and 89 and 87 and 91 supplied from corresponding phase terminals 19 and 47 and 25 and 53 of the two networks 1 and 3 are connected through common load resistors 103 and 105, respectively. Each of the load resistors 103 and 105 is coupled to the utilization apparatus 167, tor example, a modulating network such as (11 or 13) of Lenehans 1, through a coupling capacitor 111 and 113.

In accordance with the, specific aspects of my invention, the phase-splitters included in any system are designed .to operate in contiguous frequency bands. A situation may arise which may demand a series of phasesplitters designed to operate in separate bands. Systems including such phase-splitters are within the scope of the broader aspects of my invention.

While I have shown and described a certain specific object of my invention, I am fully aware that many modifications thereof are possible. My invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claims.

' I claim as my invention:

1. Broad-band single side-band apparatus capable of operating over the band between frequencies f1 and fr where fl is less than fr characterized by a plurality of phase-splitters each designed to operate over a difierent band between fr to fr-i-l, where r=l to (n-1) and fr+l is greater than ftand less than in, each phasesplitter operating substantially without distortion over its band, there being a suflicient number of such phasesplitters to cover the hand between f and in and by an electric discharge device having an input circuit and an output circuit, said input circuit of each device being associated with a corresponding phase of a corresponding one of said phase-splitters, each output first terminal of each phase-splitter being connected in the input circuit of said corresponding one of said electric discharge devices and the electric discharge devices, the inputs of which are supplied from corresponding phases of said phase-splitters, having common output loads.

2. Apparatus according to claim 1 characterized by the fact that in the input of each phase-splitter a filter designed to pass only the band (fr to fr+1 for which its corresponding phase-splitter is designed, the inputs or" all said filters being connected to a common terminal and each filter being designed to function in effect as the input component of the others.

References Cited in the file of this patent UNITED STATES PATENTS 1,550,634 Espenschied Aug. 25, 1925 1,615,252 Zobel Jan. 25, 1927 2,076,248 Norton Apr. 6, 1937 2,476,880 Lenehan .iuly 19, 1949 OTHER REFERENCES Terman: Radio Engineers Handbook, first edition, 1943, published by McGraw-Hill Book Company, New York, pages 237-238. 

